Fluidic oscillator



May 26, 1.970 c. R. HALBACH ETAL 5 FLUIDIC OSCILLATOR Filed May '7. 1968 INVENTORS CARL R. HALBACH BY LEWIS D. BICKERTON A TTORNE Y United States Patent O 3,513,868 FLUIDIC OSCILLATOR Carl R. Halbach, Van Nuys, and Lewis D. Bickerton, Reseda, Calif., assignors, by mesne assignments, to the United States of America as represented by the Umted States Atomic Energy Commission Filed May 7, 1968, Ser. No. 727,170 Int. Cl. Fc 1/08 U.S. Cl. 137-815 4 Claims ABSTRACT OF THE DISCLOSURE A fluidic oscillator having a single output channel. Frequency is made substantially independent of pressure by the use of inlet and output nozzles in a pressure divider arrangement.

BACKGROUND OF THE INVENTION This invention relates to fluidic oscillators and, more particularly, to a pressure-insensitive, temperature-dependent fluidic oscillator having a single output channel.

The invention was made in the course of, or under, United States Atomic Energy Commission Contract AT (04-3)639.

Recently developed fluidic oscillators, having no moving parts other than the fluid itself, are capable of performing functions previously performed only by electronic circuits and, to a lesser extent, by mechanical systems having moving parts. Generally in such fluidic oscillators a fluid stream is switched alternately between two output channels, the two most common switching means being stream interaction and boundary layer control. EX- amples of such oscillators and detailed analyses of the stream interaction and boundary layer control phenomena can be found in B. M. Hortons US. Pat. 3,024,805, R. W. Warrens US. 3,158,166, and US. 3,185,166 issued to B. M. Horton et al.

SUMMARY OF THE INVENTION The present invention provides a fluidic oscillator having single feedback and output channels and utilizing stream interaction and filling means for switching the fluid alternately between them. The frequency of oscillation is determined primarily by the length of the two channels and the temperature of the fluid. Proper sizing of inlet and output nozzles makes this frequency substantially independent of fluid pressure.

Accordingly, it is one object of the invention to provide a fluidic oscillator having a single output channel.

It is another object of the invention to provide a temperature-dependent, pressure-insensitive fluidic oscillator.

The invention, together with further objects and advantages thereof, can be better understood by reference to the following specification and drawing.

DESCRIPTION OF THE DRAWING The single figure of the drawing is an exploded view of one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The essential elements of the fluidic oscillator, as depicted in said figure are an inlet nozzle 11, a knife edge or flow splitter 12, a feedback channel 13, a output channel 14 and an output nozzle 15. Any compressible fluid, gas or vapor may be used as the working fluid.

A first terminal opening of said feedback loop channel 13 is arranged adjacent to one side of said knife edge 12 in a position appropriate to receive compressible fluid jet flow deflected therein from said knife edge. A second terminal opening of said feedback channel 13 is arranged on said one side of the knife edge to direct the fluid deflected into the feedback channel directly across the path of the jet flowfrom said nozzle 11 at a location between said nozzle and knife edge. Accordingly, when fluid under pressure is applied to the oscillator through inlet nozzle 11, a jet flow issues toward knife edge 12. Initially, this jet flow is divided by the knife edge and flows into both output channel 14 and feedback channel 13. Subsequently, two effects cause the jet flow to oscillate across the knife edge, alternately favoring each of the channels, thus resulting in an oscillation in pressure within the oscillator and in output flow through nozzle 15.

The two effects which cause the jet flow to oscillate across knife edge 12, i.e., to switch alternately between output channel 14 and feedback channel 13, are stream interaction and filling. Stream interaction occurs when the flow from feedback channel 13 is directed against the side of the jet flow in the region between inlet nozzle 11 and knife edge 12. If there is no splash or bounce of the two streams, momentum is conserved and the jet flow is deflected from its original direction by an angle whose tangent is a function of the momentum of the feedback flow and the original momentum of the jet flow. Thus, as flow in feedback channel 13 increases, the jet flow is deflected toward the output channel side of knife edge 12 until substantially the entire jet flow is directed to output channel 14.

Whenever the jet flow favors output channel 14, that channel tends to fill since output nozzle 15 restricts the flow therefrom. This net filling causes the pressure in the channel to increase, creating a pressure differential which pushes the jet flow back toward feedback channel 13. A similar filling eflect occurs when the feedback channel is favored, but to a lesser extent since there is no nozzle corresponding to output nozzle 15 to restrict flow from that channel. Hence, while the filling elfect alone switches the flow from the output to the feedback channel, stream interaction is the predominant effect in switching from the feedback to the output channel.

Proper proportioning of the output and feedback channels produces a stable oscillation of sinusoidal waveform with a frequency corresponding essentially to the natural frequency of the coupled channels. Although the oscillator will operate stably over a considerable rage of ratios of feedback and resonance channel lengths, optimum performance has been found to correspond to a feedback channel length of about twice the output channel length.

This optimum ratio of output and feedback channel lengths is in accord with theoretical expectations. Since the opening in output nozzle 15 is very small compared to the size of output channel 14, the output channel can be considered acoustically to be a closed pipe. Feedback channel 13, on the other hand, is open at both ends and appears acoustically to be an open pipe. Since the natural frequency of an open pipe is half that of a closed pipe of the same length, resonance should occur in the oscillator when the open pipe feedback channel is twice as long as the closed pipe output channel.

For given output and feedback channel lengths, the resonant frequency of the oscillator is proportional to the sonic velocity of the working fluid. Since the velocity of sound varies with the square root of temperature, the oscillator frequency is dependent on the temperature of the fluid. In certain applications, such as temperature sensing, this temperature dependence is desirable and hence would not be eliminated. Where variations in frequency with the temperature are undesirable, one obvious way of eliminating them would be by maintaining the fluid at a constant temperature. The oscillator may also be made temperature insensitive by utilizing a thermally expansive material for one or both of the resonance channels. When this is done, a change in fluid temperature will produce a change in the resonant frequency of the coupled channels which will oppose the undesired frequency change with temperature.

Inlet jet velocity can affect the resonant frequency of a fluidic oscillator utilizing stream interaction. While the effect on frequency of this momentum interaction is small compared to the effect of the channel lengths, some reduction in frequency sensitivity to inlet pressure is possible by controlling the inlet jet velocity.

Inlet jet velocity is proportional to the temperature of the working fluid and to the ratio of the DC. pressure within the oscillator to the pressure of the incoming fluid. When the oscillator is used as a temperature sensor, variations in inlet jet velocity with temperature are not important since this effect can be included in the oscillator frequency versus temperature calibration. In other applications, it may be necessary to maintain the fluid at a constant temperature or use a thermally expansive material to eliminate these variations.

It has been found that variations of inlet jet velocity with the pressure of the working fluid can be eliminated by maintaining the ratio of ocillator to inlet pressure at a constant value. This is done in the present invention by sizing output nozzle 15 to make the flow therefrom sonic or choked. It is not necessary for inlet nozzle 11 to be sonic to achieve pressure insensitivity. The use of a sonic nozzle in series with an upstream nozzle to generate therebetween a pressure which is proportional to the inlet pressure is called the pressure divider eflect.

By way of example, fluidic oscillator having an output channel length of 5.1 inches, an optimum frequency of 550 cycles per second, and an optimum feedback channel length between 8.4 and 10.8 inches was constructed according to the principles of the invention. With air as the working fluid, the output frequency increased from about 540 to 620 c.p.s. as temperature was raised from 550 to 750 R. The sensitivity of the output frequency to changes in input pressure was observed to be about 1% with the output flow choked or sonic, whereas it was about 3% with a subsonic or unchoked output nozzle. With an oscillator pressure recovery as high as 90 percent, a signal amplitude of about 5 percent of the inlet pressure is available.

Using the entire flow emanating from output nozzle 15 as the ouptut signal minimizes the fluid flow consumption of the oscillator. Alternatively a high impedance output signal could be obtained by tapping either the output or feedback channel.

The embodiment shown in the drawing is made of two blocks, and 20, fastened together by screws, not shown, or other suitable means. A gasket or other sealing means, not shown, can be inserted between the blocks to prevent leakage of the fluid. The particular form of construction is not critical.

What is claimed is:

1. A fluidic oscillator having a frequency of oscillation substantially independent of the pressure of the working fluid comprising:

(a) an inlet nozzle means for providing a jet flow of a compressible working fluid,

(b) knife edge flow splitter means positioned in the path of said working fluid jet flow,

(0) feedback channel loop means including a first terminal opening adjacent a first side of said knife edge in a position to receive fluid jet flow deflected therein by said knife edge and a second terminal opening directed to discharge the deflected fluid transverse to said jet flow path on said first side of the knife edge at a location between said nozzle means and said knife edge,

((1) a single output channel means having an inlet opening adjacent to the second side of said knife edge to receive fluid jet flow deflected therein by said knife edge, said output channel means including an output nozzle opening for discharging compressible fluid therefrom, and

(e) wherein said jet flow is caused to oscillate across said knife edge alternately in a first direction as caused predominantly by stream interaction between the compressible fluid discharged from the second terminal opening of said feedback channel and the jet flow from said inlet nozzle and in a second direction by filling of said output channel.

2. A fluidic oscillator as defined in claim 1 wherein the feedback channel is substantially twice as long as the output channel.

3. A fluidic oscillator as defined in claim 1 wherein the output nozzle is a sonic nozzle.

4. A fluidic oscillator as defined in claim 1, wherein the length of said feedback channel loop means is about twice the length of said output channel means.

References Cited UNITED STATES PATENTS 3,158,166 11/1964 Warren 137-815 3,159,168 12/1964 Reader 137-815 3,185,166 5/1965 Horton et al 137-815 3,204,652 9/1965 Bauer 137-815 3,217,727 11/1965 Spyropoulos 137-815 3,228,410 1/1966 Warren et al 137-815 3,238,958 3/1966 Warren et al. 137-815 3,285,262 11/1966 Ernst et al. 137-815 3,390,692 7/1968 Hastie et al. 13781.5 3,402,727 9/1968 Boothe 137-815 SAMUEL SCOTT, Primary Examiner 

